With the decreasing size of features that are fabricated in semiconductor wafers, antireflective layers are increasingly being used in photolithographic processes. An antireflective layer (ARL) is formed immediately below a photoresist layer. When the photoresist is exposed to light in the patterning process, the ARL inhibits the reflection of light from the interface between the photoresist layer and the underlying layer and thereby improves the definition of the pattern that is formed when the photoresist is developed. For a general discussion of the use of ARLs, see T. Perara, "Anti-Reflective Coatings: An Overview"; Solid State Technology, Vol. 37, No. 7; pp. 131-136 (1995), which is incorporated herein by reference.
Another aspect of decreasing feature size is the use of light radiation having a shorter wavelength. Whereas "I-line" radiation having a wavelength of 365 nm has been the standard, deep ultraviolet (UV) radiation having a wavelength of 248 nm is now being used more frequently. This transition to a shorter wavelength requires a different type of ARL. Organic layers have been commonly used as ARLs for I-line radiation, although inorganic layers are increasingly being used. Layers of inorganic materials such as silicon oxynitride are predominately used for deep UV radiation. For a discussion of the use of deep UV ARLs, see T. Ogawa et al.; "Practical Resolution Enhancement Effect By New Complete Anti-Reflective Layer In KrS Excimer Laser Lithography"; Optical/Laser Microlithography; Session VI; Vol. 1927 (1993), incorporated herein by reference. Organic ARLs are relatively thick (e.g., 1500.ANG.) and are typically spun on to the underlying substrate, while inorganic ARLs are generally thinner (e.g., 300.ANG.) and are formed by chemical vapor deposition. An advantage of the inorganic ARLs is that they generally do not need to be removed after the photolithographic process has been completed. Also, the inorganic layers tend to be less expensive than the organic layers.
One problem with inorganic ARLs such as silicon oxynitride is that amines (NH.sub.2 groups) form on the surface of the layer. The exposure of photoresists that are used with deep UV radiation generally creates an acidic reaction in the photoresist. The resulting acidic compounds then react with the developer to create a mask. Since the amines are basic, they tend to neutralize the acids generated by the exposure of the photoresist and thereby inhibit the development of the photoresist.
This problem is illustrated in FIG. 1, which shows a photoresist layer 10 and a silicon oxynitride ARL 18 overlying a substrate 12, which is to be patterned. Deep UV radiation 14 is incident on area of the top surface of photoresist layer 10, exposing a portion of the photoresist layer 10. Ideally, the exposed portion of the photoresist layer 10 is removed when photoresist layer 10 is developed, yielding the clean vertical walls denoted by the dashed lines. In practice, however, amines at the interface between photoresist layer 10 and ARL 18 neutralize some of the acids produced during exposure of photoresist layer 10 and interfere with the development process. The result is photoresist "footing" 16 which in turn interferes with the patterning of substrate 12.